This invention relates to signal processing systems and has particular reference to charge coupled device bandpass filter structures and manner of operation thereof as well as to demodulators and modems employing such bandpass filters.
Charge coupled device bandpass filters have previously been proposed, see for example U.S. Pat. No. 3,997,973 issued Dec. 21, 1976 and co-pending applications Ser. No. 758,365 filed Jan. 10, 1977 by Lawrence H. Ragan for "Programmable Frequency Converting Filter" and Ser. No. 758,366 filed Jan. 10, 1977 by Jerry L. Norris et al. for "Frequency Converting Filter", all assigned to the assignee of the present application.
Modems also are well known, being widely employed in data transmission systems for demodulation of incoming analog signals into digital mark/space data and for modulation of mark/space data in the outgoing analog signal. R. C. French in "Binary Transistor Filters in Data Modems", The Radio and Electronic Engineer, Vol. 44, No. 7, July 1974, pages 357 to 362 describes the use of binary transistor filters in modems, teaching the use of a transistor filter as a matched filter and suggesting in general terms that such filters might be implemented using charge coupled devices to implement an analog delay line.
It is an object of the present invention to provide an improved charge coupled device bandpass filter structure for filtering analog signals to provide an output signal with reduced offset voltage content.
It is also an object of the invention to provide a multi-channel bandpass filter apparatus including a plurality of charge coupled device bandpass filters all having the same structure but operable to define different passbands.
It is a further object of the invention to provide modem apparatus including charge coupled device bandpass filters.
In one aspect, the invention provides apparatus for receiving and filtering modulated analog signals having different carrier frequencies. Input terminal means is provided for receiving said analog signals on at least two frequency channels. There is an individual charge coupled device bandpass filter for each of said two channels and switching means operable to select and connect one of said bandpass filters between said input terminal means and frequency discriminator means. Each of said bandpass filter means has an identical structure and includes phase electrode sets, each said set including a split-electrode for signal amplitude weighting, the split electrodes of each filter disposed and configured to define a bandpass characteristic for said filter. Means are provided for applying phased pulses to said bandpass filter phase electrodes such that the frequencies of said phased pulses are different for each said filter and each filter has a predetermined center frequency and bandwidth. The switch lines are operated by switching pulses to select a particular bandpass filter having a center frequency matched with the carrier frequency of the analog signal received at said input terminal means.
In another aspect, the invention provides signal filtering apparatus comprising a two-phase charge coupled device bandpass filter having input means for receiving analog signal samples, the charge coupled device bandpass filter means having phase electrode sets each set including a split electrode for signal amplitude weighting, the disposition and configuration of split electrodes defining a bandpass filter characteristic for said filter. First portions of said split electrodes are connected in common to a first sampling gate; and the other portions of said split electrodes are connected in common to a second sampling gate. Clock pulses are applied to said electrode sets to shift charge between the electrode sets. First gating pulses are applied to said first and second sampling gates while analog signal charge is stored at charge storage sites beneath electrodes of said sets other than said split electrodes thereby to sample signals associated with stray capacitances coupled to said split electrodes to produce respective first and second offset voltages. Second gating pulses are applied to said first and second sampling gates while signal charge is stored beneath said split electrodes of said electrode sets thereby to sample the signal charge beneath said split electrodes to produce first and second signal related voltages each including an offset voltage, and means operable to subtract said first and second offset voltages from said first and second signal related voltages respectively to produce first and second signal voltages substantially free from offset voltage content.
The invention also provides modem apparatus including demodulator means for converting modulated analog signals received at a signal terminal means to digital mark/space signals, and modulator means for converting digital mark/space signals into modulated analog signals and transmitting said analog signals to said signal terminal means for transmission therefrom.
The demodulator has input circuit means including an individual charge coupled device bandpass filter for each of at least two data channels each selectively operable according to different transmission standards and switching means operable to select and connect one of said bandpass filters to said signal terminal means for demodulating said received analog signals. Each of said bandpass filter means has an identical structure and includes multi-phase electrode sets, each said set including a split-electrode for signal amplitude weighting, the split electrodes of each filter disposed and configured to define a bandpass characteristic for said filter. Means are provided for applying phased pulses to said bandpass filter phase electrodes such that the frequencies of said phased pulses are different for each said filter and each filter has a predetermined center frequency and bandwidth determined by the frequency of said phased signals. Switching pulses are applied to operate said switching means to select a particular bandpass filter and apply phased pulses thereto to provide a center frequency matched with the carrier frequency of an analog signal received on a particular channel at said signal terminal means.
The modulator means may comprise a plurality of capacitor means connected in series between a common output means and respective switching means to charge or discharge said capacitor means, and means for operating said switches in a selected sequence and at an adjustable rate for charging and discharging said capacitors to generate at said output means an output signal having a wave form determined by said selected sequence and a frequency determined by said adjustable rate.
In a particular embodiment, a modem is provided having full duplex capability at a 300 baud transmission rate, at both U.S.A. and C.C.I.T.T. standards, half duplex capability at a 1200 baud transmission rate, also at both U.S.A. and C.C.I.T.T. standards, together with additional low speed reverse channel modes, all such functions provided utilizing only three CCD bandpass filters selectively clocked at different rates to provide the required adjustments of bandpass characteristics. The modem structure together with the required clock generator and control circuits can be fabricated on a single semiconductor chip using, for example, current MOS process technology, which may be housed, for example, in a 24-pin plastic encapsulated package. There is thus provided a very compact integrated modem structure which may be economically produced and suited to control by a microprocessor to enable data transmission and reception in a variety of modes according to different transmission standards whether coupled directly or indirectly within acoustic coupling to a transmission line. There is thus provided a significant advance in the art compared with currently available solid state modems using individual, dedicated active element bandpass filters based on operational amplifiers.